Lecture 17. Monday, November 17, 2014

Transcription

1 Lecture 17 Monday, November 17, 2014

2 DELIMITER So far this semester, we ve used ; to send all of our SQL statements However, when we define routines that use SQL statements in them, it can make distinguishing between the end of an SQL statement within a routine and the end of an SQL statement defining the routine difficult 1

3 DELIMITER So we can use the following to define a new delimiter, $$: DELIMITER $$! Note that we do not end this statement with any delimiter We can use any delimiter we want, but be sure it is not a string of text we use somewhere else in our SQL statements or it may prematurely end and execute them When finished, don t forget to change the delimiter back with: DELIMITER ;! 2

4 DELIMITER Note, if we run the following code: DELIMITER $$ SELECT * FROM students; What happens? The prompt hangs waiting for input since ; is no longer the delimiter, it hasn t been told to execute the statement yet In this case, we either need to change the delimiter back to ; or run instead: SELECT * FROM students $$! 3

5 Functions Last week, we learned how to write functions that return a simple calculation, function call, result of an SQL statement, etc. But we may need functions that do more Suppose we want to write a function for the Sakila database that calculates how much money somebody owes (assume nobody rented the same film twice for this example) 4

6 Functions DELIMITER $$! CREATE FUNCTION moneyowed(cid INT)! RETURNS DECIMAL(5,2)! READS SQL! BEGIN! DECLARE v_spent DECIMAL(5,2);! DECLARE v_paid DECIMAL(5,2);! SELECT IFNULL(SUM(rental_price), 0) INTO v_spent FROM film WHERE film_id IN(SELECT film_id FROM inventory WHERE inventory_id IN(SELECT inventory_id FROM rental WHERE customer_id = cid));! SELECT IFNULL(SUM(amount), 0) INTO v_paid FROM payment WHERE customer_id = cid;! RETURN v_spent v_paid;! END $$! DELIMITER ;! 5

7 Functions Note that to create/initialize a variable, we use: DECLARE variable_name type_definition;!! where type_definition is similar to what we would put for the data type in a CREATE TABLE statement Then, to take a variable we are retrieving in a SELECT statement, we use INTO variable_name to assign that selected value to a variable 6

8 Procedures In other programming languages, when we want to save a group of calculations or operations that we do to be called at different points in a program, we always defined them as functions MySQL makes a distinction between these o We will use functions to perform a calculation of some sort o We will use procedures to perform a set of operations Procedures cannot be used in conjunction with any other MySQL statements such as SELECT, INSERT, UPDATE, or DELETE as functions can 7

9 Procedures With procedures, we can return multiple values Return values are not simply returned as they are with a function they are instead defined as parameters to the procedure The parameter list is the same as a function, except we need to precede each parameter with one of the following: o IN: Parameter that is sending the procedure a value that cannot be changed o OUT: Parameter that is simply for returning a value o INOUT: Parameter that both sends the procedure a value that can be changed for a return value 8

10 Procedures When we define a procedure, most of the same attributes (deterministic, SQL mode, etc.) apply For example, suppose we want to write a procedure that lists all movies in inventory at a store, and stores the number of movies in a variable 9

11 Procedure DELIMITER $$! CREATE PROCEDURE in_inventory(in sid INT, OUT titles INT)! READS SQL DATA! BEGIN! SELECT title FROM film WHERE film_id IN(SELECT film_id FROM inventory WHERE store_id = sid);! SELECT FOUND_ROWS() INTO titles;! END $$! DELIMITER ;! 10

12 Procedures Now, to use this procedure to get a list of films at store 1, we run: CALL To see how many rows were returned, we stored that in the second argument, so we can run: Note that we used FOUND_ROWS() in our procedure that is a special function we ll talk about during the next class 11

13 To delete a procedure: Procedures DROP PROCEDURE procedure_name;! To get a list of procedures we ve created: SHOW PROCEDURE STATUS WHERE Db = 'databasename';! 12

14 Triggers Triggers allow you to perform an action automatically when other statements are called It is often used for data validation Syntax: CREATE TRIGGER trigger_name time event ON tablename FOR EACH ROW action;! o time may be BEFORE or AFTER! o event may be INSERT, UPDATE or DELETE! 13

15 Triggers Suppose we have the following table definition: CREATE TABLE payments (cust_id INT, amount DECIMAL(5,2));! Suppose we want to create a trigger that tracks the total amounts that are inserted into that table:! CREATE TRIGGER amt_sum BEFORE INSERT ON payments FOR EACH ROW + NEW.amount;! We can then run the following:! = 0;! INSERT INTO payments (cust_id, amount) VALUES (1, 10.60), (1, 8.22), (2, 9.10);! It would then display

16 Triggers A few notes: o It is important to note the variable (in this used so that it can be referenced later o NEW preceding the amount field is to note that it is the NEW record being added o NEW is the only option when the event is INSERT! o OLD is the only option when the event is DELETE, and works similarly o NEW and OLD can be used when the event is UPDATE, and allows you to tell what either the original or updated value in a record was 15

17 Triggers Suppose we want to create a trigger that rounds down (floors) all values inserted into the table:! CREATE TRIGGER amt_rnd BEFORE INSERT ON payments FOR EACH ROW SET NEW.amount = FLOOR(NEW.amount);! We can then run the following:! INSERT INTO payments (cust_id, amount) VALUES (1, 10.60), (1, 8.22), (2, 9.10);! SELECT amount FROM payments;! The results would show: 10.00, 8.00,

18 Triggers Note that you can only define one trigger for each combination of a time/action Thus, if you attempted to create both of the triggers we just wrote, you would get an error But we can combine them into one: CREATE TRIGGER amt_ins BEFORE INSERT ON payments FOR EACH ROW SET NEW.amount = + FLOOR(NEW.amount);! 17

19 Triggers Now, if we run the following: = 0;! INSERT INTO payments (cust_id, amount) VALUES (1, 10.60), (1, 8.22), (2, 9.10);! SELECT amount FROM payments;! We would first get as the sum, and then we would get the values of 10.00, 8.00, and 9.00! 18

20 To delete a trigger: Triggers DROP TRIGGER trigger_name;! To get a list of triggers we ve created: SHOW TRIGGERS;! 19